This invention relates to a method for making gypsum products using a modifier and a polycarboxylate dispersant. More specifically, it relates to the making of a gypsum slurry and a gypsum panel that adds the dispersant and modifier in a specific sequence.
Gypsum-based building products are commonly used in construction. A gypsum panel made of gypsum is fire retardant and can be used in the construction of walls of almost any shape. It is used primarily as an interior wall and ceiling product. Gypsum has sound-deadening properties. It is relatively easily patched or replaced if it becomes damaged. There are a variety of decorative finishes that can be applied to the gypsum panel, including paint and wallpaper. Even with all of these advantages, it is still a relatively inexpensive building material.
One reason for the reasonable cost of gypsum panels is that they are manufactured by a process that is fast and efficient. A slurry, including calcium sulfate hemihydrate and water, is used to form the core, and is continuously deposited on a paper cover sheet moving beneath a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with a sufficient amount of the water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture. Since each of these steps takes only minutes, small changes in any of the process steps can lead to gross inefficiencies in the manufacturing process.
The amount of water added to form the slurry is in excess of that needed to complete the hydration reactions. Some of the water that is added to the gypsum slurry is used to hydrate the calcined gypsum, also known as calcium sulfate hemihydrate, to form an interlocking matrix of calcium sulfate dihydrate crystals. Excess water gives the slurry sufficient fluidity to flow out of the mixer and into the facing material to be shaped to an appropriate width and thickness. While the product is wet, it is very heavy to move and relatively fragile. The excess water is removed from the board by evaporation. If the excess water were allowed to evaporate at room temperature, it would take a great deal of space to stack and store the gypsum panel while it was allowed to air dry or to have a conveyor long enough to provide adequate drying time. Until the board is set and relatively dry, it is somewhat fragile, so it must be protected from being crushed or damaged.
To dry the boards in a relatively short period of time, the gypsum panel product is usually dried by evaporating the extra water at elevated temperatures, for example, in an oven or kiln. It is relatively expensive to build and operate the kiln at elevated temperatures, particularly when the cost of fossil fuel rises. A reduction in production costs could be realized by reducing the amount of excess water present in set gypsum boards that is later removed by evaporation.
Another reason to decrease water is that the strength of gypsum products is inversely proportional to the amount of water used in the manufacturing of full density products. As the excess water evaporates, it leaves voids in the matrix once occupied by the water. Where large amounts of water were used to fluidize the gypsum slurry, more and larger voids remain in the product when it is completely dry. These voids decrease the product density and strength in a finished product such as poured flooring.
Dispersants used with gypsum help fluidize the mixture of water and calcium sulfate hemihydrate so that less water is needed to make a flowable slurry. Naphthalene sulfonate dispersants are well known, but have limited efficacy. Polycarboxylate dispersants are commonly used with cements and, to a lesser degree, with gypsum. The class of compounds represented by the term “polycarboxylate dispersants” is huge, and it is very difficult to predict how individual compounds react in different media.
Modifiers can be added to a gypsum slurry to increase the efficacy of the dispersant. Salts, including silicates and carbonates, are particularly effective modifiers. More specifically, quicklime, lime and soda ash are especially effective with polycarboxylic ether dispersants. Modifiers are disclosed in greater detail in copending U.S. Ser. No. 11/152,317, entitled, “Modifiers for Gypsum Products and Methods of Using Them”, previously incorporated by reference.
It has been found, however, that the methods used to make mixtures of gypsum, dispersants and modifiers do not consistently produce highly fluid slurries. In some instances, the efficacy of the dispersant is dramatically increased, but in other cases, the same combination of components had little effect on the fluidity of the slurry. This inability to predict the fluidity of the slurry can lead to increased dispersant use to ensure that a minimum fluidity is achieved. In a process where the dispersant can be one of the most expensive components, dispersant overdose unnecessarily increases the product cost at a rapid rate.
Another property of polycarboxylate dispersants is their tendency to retard the set of a gypsum slurry. If high doses of dispersants are used to accommodate uncertain dispersant efficacy, set may be retarded to the degree where certain products, such as the gypsum panel, cannot be produced on current, high-speed equipment.
Thus, there is a need for a method to make a gypsum slurry, and products therewith, that consistently generate a highly fluid mixture without excessive dosages of dispersants. Use of such a method would keep the cost reasonable where expensive dispersants were used, and would minimize set retardation of the slurry.